Prototype of a Negative-Sequence Turn-to-Turn Fault Detection Scheme for Transformers

Digital relays are capable of computing the negative-sequence current on both primary and secondary sides of the transformer along with the phase difference between these two negative-sequence currents. By using both phase and magnitude information, negative-sequence current could be used to detect minor turn-to-turn faults involving 3% of the transformer's windings or more. Turn-to-turn faults may still occur even if no current is flowing on one side of the transformer, such as during energization. With no current flowing in the secondary windings of the transformer, negative-sequence current-based algorithms become insensitive. This paper introduces a relay prototype, using both negative-sequence current and negative-sequence voltage, which retains its sensitivity during energization. The relay's performance for several commonly encountered system scenarios, such as overexcitation, current-transformer saturation, nonzero fault resistance, transformer energization, and external faults were also examined. The experimental results presented in this paper indicate that the algorithm proposed in this paper is faster and more sensitive than restrained current differential protection and is capable of detecting turn-to-turn faults occurring during transformer energization.